The present invention relates to apparatus and methods for qualifying signal levels. More specifically, the present invention provides apparatus and methods for enabling and disabling an amplifier based on input signal levels. Still more specifically, the present invention provides apparatus and methods for comparing an amplified input signal to threshold levels in order to determine whether to enable or disable an amplifier.
Many applications require the referencing of input signals. Input signals can be compared to particular thresholds and the results of these comparisons can be used to determine output signals. In one particular situation, input signals can be referenced to determine whether to enable or disable an amplifier. When input signals are sufficiently small, for example, an amplifier can be disabled to save power and limit heat dissipation.
Small input signals require amplification prior to referencing by conventional comparators. The addition of a stage of amplification, however, adds noise, distortion, offset, drift, and possibly other undesirable effects. Typical workarounds include the use of common-centroid layouts and differential pairs. Components with identical electrical and thermal properties can be wired symmetrically such that drafts and linear gradients arising during their operation cancel. Such fluctuations in their characteristics are fully compensated by their symmetrical components. Common-centroid layouts are effective in reducing noise and thermal drift. However, long-term drift and non-linear gradients remain significant problems.
The chopper technique is another workaround used to limit undesirable amplification effects. DC signals are converted into periodic signals at a known frequency. The periodic signals are amplified and subsequently demodulated by multiplying the amplified signal with the signal used to initially convert the signal into a periodic one. This technique provides for low long-term drift. However, as signal frequencies approach the clock frequency, the chopper amplifier can introduce aliasing. Chopper amplifier systems thus have typically been limited to low frequency applications. Chopper amplifiers also tend not only to be more expensive than traditional precision amplifiers, but they can also introduce substantial noise in the form of large ripple voltages resulting from voltage swings at the predetermined clock intervals.
A variation on chopper amplifier is the chopper-stabilized amplifier. Two operational amplifiers are placed on a chip, along with offset-error storage capacitors. The main operational amplifier functions as a conventional amplifier. The nulling operational amplifier monitors the input offset of the main operational amplifier. Operation functions in alternating stages. In the first stage of operation, the nulling operational amplifiers inputs are shorted and the amplified feedback is used to eliminated offset error of the nulling operational amplifier. In the second stage of operation, the inputs of the nulling operational amplifier are connected to the inputs of the main operational amplifier so that the main operational amplifier now has zero offset. However, the chopper-stabilized amplifier is susceptible to significant amounts of digital switching noise as it cycles between stages of operation.
It is therefore desirable to provide improved techniques and systems for enabling or disabling an amplifier based on referenced input signals.
According to the present invention, methods and apparatus are provided for qualifying signal levels. Mechanisms are provided for enabling and disabling an amplifier based on a determination of the input signal level. The amplifier can be disabled primarily for saving energy and reducing heat dissipation. In one particular implementation, input signals from multiple channels in a variety of possible configurations can be amplified using a conventional gain stage. The amplified signal can then be compared with threshold voltages using threshold circuitry. Depending on the output of the threshold circuitry operations, the detector circuit may send an enable or disable signal to the amplifier. The output of the threshold circuitry can also adjust the amplified signal level to oppose drift in the gain stage.
One aspect of the invention provides apparatus for enabling and disabling an amplifier that has an associated signal level. The circuit can be characterized by the following features: (1) a gain stage for amplifying the signal level and generating an amplified signal level; (2) threshold circuitry for comparing the amplified signal level to a plurality of thresholds and indicating a signal level state representative of the amplified signal level; and (3) control circuitry for enabling and disabling the amplifier in response to the signal level state, the control circuitry comprising adjustment circuitry coupled between the threshold circuitry and the gain stage for introducing discrete signal level adjustments to the amplified signal level in opposition to a change in the amplified signal level indicated by a corresponding change in the signal level state, thereby opposing drift in the amplified signal level.
The control circuitry may be used to determine whether to adjust the discrete value. In one embodiment of this invention, breaches of the same threshold detector must occur on consecutive clock cycles before the discrete value is changed. In one example, the discrete value may be converted into an analog signal level adjustment using a digital to analog converter.
Another aspect of the invention provides apparatus for enabling and disabling an amplifier that has an associated signal level. The circuit can be characterized by the following features: (1) a gain stage for amplifying the signal level and generating an amplified signal level; (2) threshold circuitry for comparing the amplified signal level to a plurality of thresholds and indicating a signal level state representative of the amplified signal level; and (3) control circuitry for enabling and disabling the amplifier in response to the signal level state, the control circuitry comprising adjustment circuitry coupled between the threshold circuitry and the gain stage for introducing signal level adjustments in opposition to a change in the amplified signal level indicated by a corresponding change in the signal level state, thereby preventing enabling of the amplifier by the control circuitry where the change in the signal level state is reversed by the introduction of signal level adjustments.
Yet another aspect of the invention provides apparatus for enabling and disabling an amplifier that has an associated signal level. The circuit can be characterized by the following features: (1) a gain stage for amplifying the signal level and generating an amplified signal level; (2) threshold circuitry for comparing the amplified signal level to a plurality of thresholds and indicating a signal level state representative of the amplified signal level, the plurality of thresholds comprising a first threshold corresponding to a first value of the amplified signal level, and a second threshold corresponding to a second value of the amplified signal level greater than the first value; and (3) control circuitry for enabling and disabling the amplifier in response to the signal level state, the control circuitry comprising qualification circuitry for qualifying enablement of the amplifier where the amplified signal level is between the first and second values, the control circuitry being operable to enable the amplifier without regard to the qualification circuitry where the amplified signal level exceeds the second value.
Another aspect of the invention provides apparatus for enabling and disabling an amplifier that has an associated signal level. The circuit can be characterized by the following features: (1) a gain stage for amplifying the signal level and generating an amplified signal level; (2) threshold circuitry for comparing the amplified signal level to a plurality of thresholds and indicating a signal level state representative of the amplified signal level; and (3) control circuitry for enabling and disabling the amplifier in response to the signal level state, the control circuitry comprising adjustment circuitry coupled between the threshold circuitry and the gain stage for introducing signal level adjustments in opposition to a change in the amplified signal level indicated by a corresponding change in the signal level state, and stabilization circuitry for preventing further introduction of signal level adjustments for a predetermined period of time following the introduction of a first signal level adjustment to thereby allow settling of the gain stage.
Another aspect of the invention provides apparatus for enabling and disabling an amplifier that has an associated signal level. The circuit can be characterized by the following features: (1) a gain stage for amplifying the signal level and generating an amplified signal level; (2) threshold circuitry for comparing the amplified signal level to a plurality of thresholds and indicating a signal level state representative of the amplified signal level; and (3) control circuitry for enabling and disabling the amplifier in response to the signal level state, the control circuitry comprising stabilization circuitry for preventing enabling of the amplifier for a predetermined period of time following disabling of the amplifier regardless of the amplified signal level.
Still another aspect of the invention provides apparatus for enabling and disabling an amplifier that has an associated signal level. The circuit can be characterized by the following features: (1) a buffering stage for combining the first and second signal levels to generate a combined signal level, unequal gains being applied to the first and second signal levels to generate the combined signal level; and (2) threshold circuitry for determining whether the combined signal level exceeds predetermined threshold levels and indicating a signal level state for enabling or disabling an amplifier.
A further understanding of the nature and advantages of the present invention may be realized by reference to the remaining portions of the specification and the drawings.